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This invention relates generally to electrostatic precipitators and in particular to precipitators in which the gas flows horizontally through the precipitator.
The use of electrostatic precipitators to remove suspended particles (i.e. dust) from gas flows is a well-known art and dry horizontal flow precipitators are commonly in service for such application.
The treatment zone of a common dry horizontal flow precipitator typically consists of a plurality of high voltage electrodes and collecting electrodes arranged to form gas passages that run parallel to the gas flow. The electrodes are enclosed in a casing through which the gas flows. The electrodes collect dust from the gas flow and rapping systems are used to dislodge the dust from the electrodes, which then falls due to gravity to the bottom of the casing. The bottom of the casing is equipped with a dust removal system to discharge the collected dust.
The gas flow is supplied to the precipitator through an inlet nozzle that connects an inlet duct to the casing inlet face and the gas flow is exhausted from the precipitator through an outlet duct that connects the casing outlet face to an exhaust duct.
In prior art precipitators, all of the gas flow treated by the precipitator enters the precipitator at the inlet face, flows through the precipitator treatment zone, and leaves the precipitator at the outlet face.
Optimum performance had been previously associated with uniform gas flow through the treatment zone. Absolutely uniform flow is not achievable and the Institute of Clean Air Companies, an association that includes major suppliers of electrostatic precipitators and which was formed to encourage improvement of engineering and technical standards, updated and reissued their standard ICAC-EP-7 in January 1997 to specify uniformity within acceptable limits.
The mathematical analysis leading to the conclusion that uniform gas flow provides best performance is based on the assumption that at any point within the precipitator the dust concentration does not vary from the top to the bottom of the treatment zone. Standards calling for uniform gas flow are also based on this assumption. However, this assumption is not correct, as re-entrainment of falling dust causes an ever-increasing difference in the dust concentration at the bottom of the treatment zone relative to the top as the gas flows through the treatment zone, with the result that at the precipitator outlet face, the dust concentration in the gas flow leaving the lower part of the precipitator treatment zone is much higher than from the upper part.
This was recognized in U.S. Pat. No. 3,733,785 to Gallaer and U.S. Pat. No. 4,695,297 to Hein that describe controlled non-uniform gas flows that improve precipitator collection efficiency over the uniform flow model. In the devices described in these patents, the gas flow remains entirely contained in the treatment zone from the inlet face to the outlet face.
The present invention involves a significant departure from the accepted theory and practice of containing all of the gas flow entering the precipitator in the casing through the entire treatment zone and exhausting it all at the outlet face of the precipitator.
As the gas flow passes through the treatment zone, the dust concentration in the gas flow becomes lower and reaches an average exit concentration at the outlet face. However, the dust concentration in the gas flow at the top of the precipitator is much lower than that of the gas flow at the bottom (because of re-entrainment) and the upper gas flow becomes cleaner than the average exit concentration before it reaches the outlet face.
A precipitator according to the invention extracts a portion of the gas flow (herein referred to as the xe2x80x9cextraction flowxe2x80x9d) from the main gas flow. The extraction flow is taken from the upper region of the treatment zone before the outlet face. The extraction flow can then be reintroduced to the remaining gas flow after the remaining gas flow passes through the outlet face.
This results in at least two benefits. First, extracting a portion of the gas flow from the top of the precipitator encourages the rise of the remaining gas flow as the remaining gas flow proceeds toward the outlet of the precipitator and thereby improves the efficiency of the precipitator upstream of the point of extraction by reducing re-entrainment. Second, the velocity of the remaining gas flow after the point of extraction is reduced as the volume of the gas flow has been reduced by the amount of the extraction flow. Precipitator performance depends on treatment time which is the length of time the gas flow is in the treatment zone. Reducing the velocity of the gas flow increases the treatment time and therefore raises the precipitator collection efficiency after the point of extraction of the extraction flow.